1. Field of the Invention
The present invention relates to a digital information reproducing apparatus using rotary magnetic heads that reproduces digital information signals using magnetic heads installed on a rotary drum, in particular, using Magneto Resistive Heads.
2. Description of the Related Art
The recording capacity of digital storage, including digital video tape recorders (VTR), hard disks, and optical discs, is becoming larger and larger. To increase the capacity, the recording rate is increased and the recording track width is reduced. As a reproducing head, a Magneto Resistive Head (hereinafter called an MR head) is used to maintain an S/N margin compatible with high-density recording.
An MR head, which detects a change in the resistance to sense magnetic information signals generated from a recording medium such as a magnetic tape or a magnetic disk, needs to have a detection current (sense current) supplied. Because the change in resistance is non-linear with respect to the input magnetic field, a bias current is required to keep the operation point of the MR head in an area where the change becomes almost linear. Recent MR heads are designed to receive those currents as a common current (hereinafter, those currents are collectively called a bias current). When this MR head is applied to a rotary head apparatus, it is required to install a bias current circuit, such as the one described above, and a reproduction amplifier circuit on the rotary drum.
The technology for installing an MR head on a rotary drum and controlling the bias current determining the operation point of the MR head is disclosed, for example, in the JP-A-10-177924. The publication discloses a technology for changing the bias current according to the degree of wear on the MR head.
The conventional technology described above does not consider compatibility required to reproduce data from media recorded at different densities. That is, a magnetic disk unit, in which magnetic disks and magnetic heads are integrally configured, does not assume the reproduction of data from a disk medium on which data was recorded by other units. On the other hand, a magnetic tape unit can reproduce data from a tape medium on which data was recorded by some other unit. However, a magnetic tape unit compatible with high-density recording must be able to reproduce data from a magnetic tape on which data was recorded by a conventional low-density unit.
Because the magnetic layer of a high-density recording tape is thin to improve the characteristic in the short wavelength area, the level of a signal reproduced from a high-density recording tape is generally low as compared with that of a signal reproduced from a conventional low-density recording tape. In addition, the reproduction amplifier circuit must be used in a high-gain area to minimize the effect of noise components of the amplifier itself and external noises. Therefore, when a low-density recording tape is reproduced on a unit designed for a high-density recording tape, the level of the reproduced signal becomes too high and the dynamic range of the reproduction amplifier circuit is exceeded. This involves waveform clipping or secondary harmonic distortion, resulting in degradation in the system error rate. In addition, even if an equivalent-density tape is used, the output level becomes too high when the MR head has worn; in this case, the problem similar to those described above occurs.